Apparatus and method for a telephony gateway

ABSTRACT

A system for connecting a circuit network with a packet network. The system includes a packet switch fabric, a circuit network server, and a packet network server. The circuit network server has a first and second port. The first port of the circuit network server sends and receives circuit-based signals with the circuit network. The second port of the circuit network server sends and receives packet-based signals having packets with the packet switch fabric. The circuit network server also includes a plurality of digital signal processors. A digital signal processor performs packet adaptation and subsequent to the packet adaption a second at least one digital signal processor performs signal processing. The packet network server has a first port for sending and receiving packet-based signals with the packet switch fabric and a second port for sending and receiving packet-based signals with the packet network. The packet switch fabric is capable of transferring packet-based signals among the packet network server and the circuit network server, and among the circuit network server and a second circuit network server.

PRIORITY

[0001] This application is a continuation of U.S. application Ser. No.09/127,223 filed Jul. 31, 1998, the disclosure of which is incorporatedherein by reference.

TECHNICAL FIELD

[0002] The present invention relates to network interfaces and, morespecifically, to gateways connecting circuit networks and packetnetworks.

BACKGROUND ART

[0003] Currently, circuit switching technology forms the basis for theworld-wide telecommunications network infrastructure and is usedextensively in telephone systems, however the recent expansion of theInternet has fueled the use of packet-based technologies. Packet-basedtechnologies can be used as an alternative or in combination withcircuit switching technologies in these telecommunications networks andtelephone systems. When packet-based and circuit-based communicationtechnologies are used together, a bridge, known as a gateway, isnecessary to transform and route signals between a circuit network and apacket network. Telephony gateways interconnecting to the circuitnetwork may use standards-based time division multiplexed (TDM) trunks(T1, T3, E1, etc) and standards-based signaling mechanisms (e.g.,Signaling System 7 or channel associated signaling). An example of acircuit network is the telephone system that provides subscribers withplain old telephone service (POTS). The gateway may interconnect to thepacket network through standards-based packet interfaces such asInternet Protocol (IP), Frame Relay and Asynchronous Transfer Mode (ATM)over a variety of physical interfaces (e.g., 100 BaseT, T3, OC3c,OC12c). An example of a packet network is the Internet.

[0004]FIG. 1 illustrates the architecture of a prior art telephonygateway 10. This gateway architecture uses a circuit switch fabric 12such as a TDM bus or a Time-Slot-Interchange to provide the internalswitching between the circuit network 14 and the packet network 16.Circuit-to-circuit calls, as indicated by line 15, are switched betweencircuit network servers 18 using the circuit switch fabric 12.Circuit-to-packet calls, as indicated by line 17, are switched betweencircuit network servers 18 and packet network servers 19 using this samecircuit switch fabric 12. The conversion of the circuit data to packetdata, which is known as packet adaptation, is performed in the packetnetwork servers 19, which incorporate digital signal processors (notshown) for echo cancellation and transcoding. The circuit switch fabric12, however, limits the overall flexibility of the gateway to movepackets among server cards.

[0005]FIG. 2 illustrates the architecture of another prior art telephonygateway 20. For greater flexibility, this gateway architecture separatesthe signal processing functions from the packet network servers 22 andplaces these functions on signal processing servers 21. In addition, apacket switch fabric 23 allows connectivity from any signal processingserver 21 to any packet network server 22. It is well known that apacket switch fabric 23 can be implemented with a variety oftechnologies, such as an arbitrated packet bus or a centralizedswitching module. As in the gateway architecture of FIG. 1,circuit-to-circuit calls are switched via the circuit switch fabric 12as indicated by line 24. Circuit-to-packet calls, as indicated by line25, are first switched by the circuit switch fabric 26 to a signalprocessing server 21 that contains an available digital signal processor(DSP) for performing signal processing. The signal processing server 21uses the packet switch fabric 23 to move the processed information to apacket network server 22 and the associated packet network interfaceselected during call establishment. The separation of the signalprocessing function on separate servers allows a call-by-call selectionof different DSP-based functions. For example, different calls can usedifferent compression algorithms, with differing processing complexity,residing on different signal processing servers 22. While theflexibility of the packet switch fabric 23 represents an improvementover the architecture of FIG. 1, the architecture of FIG. 2 carries thecost and complexity burden of two separate and independent switchfabrics: one circuit and one packet.

SUMMARY OF THE INVENTION

[0006] The invention provides, in a preferred embodiment, a system forconnecting a circuit network with a packet network. In one embodiment,the system contains a packet switch fabric, a circuit network server, apacket network server and a signal processing server. The circuitnetwork server can send and receive circuit-based signals with thecircuit network and can also send and receive packet-based signals withthe packet switch fabric. The circuit network server has a digitalsignal processor which provides packet adaptation. The packet networkserver can send and receive packet-based signals with the packet switchfabric and can send and receive packet-based signals with the packetnetwork. The signal processing server can send and receive packet-basedsignals with the packet switch fabric and has a digital signal processorfor performing signal processing on the packet-based signals. The packetswitch fabric transfers packet-based signals among the packet networkserver, the signal processing server, and the circuit network server.

[0007] In further embodiments, the digital signal processor of thecircuit network server performs signal processing and it may alsoperform echo cancellation. In an alternative embodiment, the circuitnetwork server may contain additional digital signal processors, whereinpacket adaptation is performed on a circuit-based signal by a digitalsignal processor forming a packet-based signal prior to signalprocessing being performed on the packet-based signal. In anotherembodiment, the digital signal processor of the signal processing serverperforms transcoding and additionally, may perform echo cancellation.

[0008] In alternative embodiments, the packet switch fabric may be aswitching module, a packet bus, or a cell bus.

[0009] In yet another embodiment, the system may further include amanagement server which is coupled to the packet switch fabric andprovides management of gateway resources.

[0010] In a related embodiment in accordance with the invention, thecircuit network server contains a line interface unit and a framer forinterfacing with the circuit network and a packet bus interface forinterfacing with the packet bus and distributing a packet-based signalto the packet bus. The packet bus interface may contain a multiplexercoupled to the digital signal processor for sending and receivingpackets. In yet another related embodiment of the invention, the signalprocessing server contains a packet bus interface and a digital signalprocessor. The digital signal processor may be configured to transcodepacket-based signals and the packet bus interface may contain amultiplexer.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011] The foregoing features of the invention will be more readilyunderstood by reference to the following detailed description, takenwith reference to the accompanying drawings, in which:

[0012]FIG. 1 is a block diagram of the architecture of a prior arttelephony gateway.

[0013]FIG. 2 is a block diagram of the architecture of another prior arttelephony gateway.

[0014]FIG. 3 is a block diagram of a system for connecting a circuitnetwork with a packet network in an embodiment in accordance with theinvention.

[0015]FIG. 4 is a block diagram of the components of a circuit networkserver in accordance with an embodiment of the invention.

[0016]FIG. 5 is a block diagram of the components of the signalprocessing server in accordance with an embodiment of the invention.

[0017]FIG. 6 is a block diagram of the components of the packet networkserver in accordance with an embodiment of the invention.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

[0018] The word “packet” as used herein defines a block of data with aheader. The term packet includes cells. A packet header typicallyincludes information, such as, the source and destination addresses or aconnection identifier. The header is used to direct the packet throughthe packet network. The term “packet switch fabric” as used hereinrefers to any device which contains the means to transfer packetsbetween two or more devices. A packet switch fabric may be, but is notlimited to, a packet bus, a switching module, a cell bus, a crossbarswitch, a space division switch or a signal router. The term“multiplexer” shall refer to any device, which may perform multiplexing,demultiplexing, or both multiplexing and demultiplexing functions. Theterm “transcoding” refers to the process of transforming a signal fromone state of coding to another. For example, an uncompressed signal maybe transcoded via a GSM audio compression technique forming a GSMencoded signal. ADPCM, LD-CELP, CELP, LPC10, CELP G.711, G.722, G.723.1,G.726, G.728, and G.729 are some examples of coding states that a signalmay be transcoded between. The term “circuit-based signal” refers to adata stream in a time division multiplexed path containing digitalinformation. The term “packet-based signal” refers to a data streamcontaining packets, wherein the packets contain digital information. Theterm, “packet adaptation” refers to the process of segmenting acircuit-based digital signal composed of samples and creating a packetfrom the segment by adding a header. Packet adaptation also refers tothe process of removing the header information from a packet andreassembling the packets to recreate the circuit-based digital signal.Packet adaptation may further include the process of time stamping.Hereinafter both special purpose digital signal processors and generalpurpose digital signal processors shall be referred to as digital signalprocessors (DSPs). The term “port” shall refer to any input or output. Aport may include multiple inputs and multiple outputs. The term “gatewaysignal processing” refers to signal processing that is performed on agateway such as transcoding, echo cancellation, silence detection,comfort noise generation, tone detection and generation, and FAX andmodem relay.

[0019]FIG. 3 shows a gateway 30 for connecting a circuit network with apacket network using a packet switch fabric in accordance with oneembodiment of the invention. This system may perform circuit-to-circuitconnections 37, circuit-to-packet connections and packet-to-circuitconnections 38, and packet-to-packet connections 39.

[0020] In one embodiment of the invention, the system of FIG. 3 iscomposed of multiple circuit network servers 33 and multiple packetnetwork servers 36 and at least one signal processing server 35 allcoupled to a packet switch fabric 34. Each server may be designed as acombination of integrated circuits and other components and placed on anindividual integrated circuit card or module for insertion into a modulereceptor board. The cards may be combined together to form differentmodules, such as, combining the packet network server 36 and the signalprocessing server 35 cards into a single module. The packet switchfabric 34 may also be implemented as a module when the packet switchfabric takes the form of a signal switcher, a router, or a packet buswith interface circuits.

[0021] In an embodiment, the system includes a circuit network server 33for receiving circuit-based signals from the circuit network 14. Inaccordance with an embodiment of the invention, each circuit networkserver 33 performs packet adaptation on a circuit-based signal to form apacket-based signal. Additional signal processing functions may also beincluded in the circuit network server such as echo cancellation. Inalternative embodiments, multiple DSPs may be located on the circuitnetwork server 33 where each DSP may perform both packet adaptation andsignal processing, just signal processing or just packet adaptation.

[0022] When the circuit network server 33 converts the arrivingcircuit-based signal such as pulse code modulated (PCM) samples intopackets, it may be necessary or desirable to perform echo cancellationon the PCM samples. In the circuit network server 33, the added delayrelated to accumulating the PCM samples to form packets could result indegraded voice quality experienced by the subscribers due to noticeableechoes. This and all other signal processing functions can be providedby specialized devices such as customized integrated circuits or generalpurpose digital signal processors in communication with processorcontrol software in the gateway. Alternatively, echo cancellation may beperformed on one of the signal processing servers 35. In one form ofimplementation, once signal processing has been performed on the samplesof the circuit-based signal, the samples are grouped into packets andheader information is attached to the packet for identifying at leastthe packet size, its source, its destination, and its numerical positionwithin the signal stream.

[0023] The packet-based signal may be transferred from the circuitnetwork server 33 to either a signal processing server 35 or directly toa packet network server 36. The signal processing server 35 providesadditional signal processing for a connection, such as transcoding,digital filtering, or echo cancellation. This process may take place onone or more DSPs within the signal processing server 35. In accordancewith one embodiment of the invention, each DSP on the signal processingserver 35 may be so equipped as to perform a separate type oftranscoding. For example, one DSP might transcode G.711 to G.729 andanother DSP might transcode G.711 to G.723.1, wherein the signal wouldbe routed to the appropriate DSP for a given connection. The signalprocessing server 35 is designed in such a way as to permit the signalto be routed between DSPs. It should be apparent to those skilled in theart that signal processing such as transcoding may be performed on apacket-based signal wherein a header of a packet in the packet-basedsignal is stripped, signal processing is performed on the data of thepacket and a header is added to the signal-processed data forming a newpacket. For some signal processing applications involving packets, itmay be necessary to accumulate multiple packets before signal processingcan be performed on data within the packets. In the course of the signalprocessing on the signal processing server 35, information within thestream of packets may be reconstituted. For example, in the case oftranscoding, data from two or more packets may be compressed and placedinto a single packet. After signal processing has been performed on thepacket-based signal in the signal processing server 35, the packet-basedsignal may be transferred to the packet network server 36. The packetnetwork server 36 performs all the functions that are necessary fortransferring the packet-based signal to the packet network 16.

[0024] Gateway functions such as the routing of the circuit-based andpacket-based signals, assigning the appropriate DSP for transcoding androuting of the signals through the gateway may be distributed throughoutthe gateway on each of the servers.

[0025] An alternative embodiment in accordance with the inventionincludes a management server 71 coupled to the packet switch fabric 34.The management server 71 has overall responsibility for the managementof gateway resources including routing of the signals and assignment ofthe appropriate DSPs. The management server 71 coordinates the overalloperation of the telephony gateway 30, including the booting of thegateway on power-up, configuration of the gateway resources, recoveryfrom component failures, and reporting of events, alarm and billinginformation to an external network management system (not shown).

[0026] The packet switch fabric 34 transfers packet-based signals amongpacket network servers 36, signal processing servers 35, and circuitnetwork servers 33. In an embodiment of the invention, the packet switchfabric 34 may be a packet bus. In another embodiment, the system mayoperate on ATM cells and the packet switch fabric 34 would be a cellbus. Packet network servers 36 and signal processing servers 35 would beconfigured to handle cells in such an embodiment.

[0027] In one embodiment in which the packet switch fabric 34 isimplemented as a packet bus, each circuit network server 33 (see FIG. 4)contains line interface units (LIUs) and framers 41 required tointerface to the circuit network 14. The circuit network server 33 mayhave multiple ports for connecting to the circuit network 14 and as aresult multiple LIUs and framers 41 and DSPs 42. In an embodiment of theinvention, the LIU converts the incoming voltage level to digital onesand zeros while the framer locates each frame of a circuit-based signal.For example, if the circuit network is a T1 connection, the framerlocates each 193-bit frame from the T1 line and extracts the 24 channelsthat make up the T1 transmission. The output of the framer may couple toa TDM-style interface on a DSP 42. The DSP may be, for example, a DSPfrom Texas Instruments, model number TMS320C549. The DSP performs atleast packet adaptation. Packet adaptation includes grouping togethersamples of a circuit-based signal, reordering packet-based signals andadding or removing a header such as an INTERNET protocol (IP), userdatagram protocol (UDP), or real-time protocol (RTP) header. In oneembodiment, the RTP header is mapped to the packet in the DSP and thenUDP and IP headers are added on by the packet network server 36. Thepackets exiting the DSPs 42 are multiplexed in the packet bus interface44 by a packet multiplexer, and sent across the packet bus 45 to eithera signal processing server 35 for transcoding, directly to a packetnetwork server 36 when transcoding is not necessary, or to anothercircuit network server 33. The packet bus interface 44 may beimplemented with a chip from TranSwitch, model number TXC-05802 incombination with a multiplexer that may be implemented with a MakerMXT3010 programmable cell processing engine. The packet bus interface 44contains all the necessary functions for implementing inlet queuing ofthe packets, cell address translation, cell routing, and outlet cellqueuing. The packet bus provides the packet switch fabric 34 of FIG. 3in this embodiment.

[0028] Access to the signal processing server 35 (see FIG. 5) is via thepacket bus 45 in one embodiment of the invention. The signal processingserver 35 contains DSPs 52, which have packet interfaces (not shown).The DSPs 52 receive packets from a packet bus interface 54 whichcontains a multiplexer for performing packet demultiplexing on theincoming packets. After the DSPs process the data, the DSPs send packetscontaining processed data to the packet bus interface 54 for packetmultiplexing. The packet bus interface 54 may be implemented with a chipfrom TranSwitch, model number TXC-05802, in combination with amultiplexer that may be implemented with a Maker MX13010 programmablecell processing engine. Packets from the packet-based signals may betransferred to any of the DSPs 52 via the packet bus interface 54. Thepacket bus interface 54 takes the multiplexed packet-based signal anddirects the signal onto the packet bus 45.

[0029] In an embodiment of the invention, each packet network server 36(see FIG. 6) interfaces to the packet bus 45 via a packet bus interface61 for sending and receiving packets to the circuit network servers 33and signal processing servers 35, and interfaces to the packet network16 by standard packet network interfaces such as Ethernet. The packetnetwork server 36 performs the packet switching functions of addresslookup and packet forwarding 62. The address lookup and packet forwarder62 may analyze the packet header to identify the necessary resources forthe connection and may strip the IP and UDP header and insert aninternal gateway connection identifier. An Ethernet Medium AccessControl (MAC) device 63 controls access to the packet network interface.A physical interface 64 or port provides the connection between a linein the packet network and the gateway. The physical interface 64 may be,but is not limited to, a coaxial interface, or a twisted pair interfacefor 10-base-T or 100-base-T connections.

[0030] Referring to FIG. 3, a circuit-based signal may be traced fromthe circuit network to the packet network as follows. A circuit-basedsignal is received from the circuit network 14 into one of the circuitnetwork servers 33. According to one embodiment of the invention, echocancellation is performed in the circuit network server 33 on thecircuit-based signal. This is followed by packet adaptation in thecircuit network server 33 to form a packet-based signal. Thepacket-based signal is transferred to the packet switch fabric 34 androuted to either one of the signal processing servers 35 if transcodingis necessary or if transcoding is not necessary to one of the packetnetwork servers 36. If transcoding is necessary, the packet-based signalis transferred from the packet switch fabric 34 to a signal processingserver 35 where transcoding occurs. Then the transcoded packet-basedsignal may be transferred directly to a packet network server 36 if thesystem is designed with such a routing connection or it is transferredback to the packet switch fabric 34 and then onto a packet networkserver 36. The packet network server 36 transfers the packet-basedsignal to the packet network 16.

[0031] It is also possible to have a packet-based signal in the packetnetwork 16 transferred to the circuit network 14 using the sameembodiment of the system as shown in FIG. 3. A packet-based signal isreceived from the packet network 16 into one of the packet networkservers 36. The packet-based signal is transferred from the packetnetwork server 36 to a packet switch fabric 34 or directly to a signalprocessing server 35 if the system is so configured. From the packetswitch fabric 34, the packet-based signal may be transferred to a signalprocessing server 35 if the packet-based signal requires transcoding. Iftranscoding is not required, the signal is transferred to one of thecircuit network servers 33. On the circuit network server 33 packetadaptation occurs transforming the packet-based signal into acircuit-based signal and then the circuit network server 33 performsecho cancellation. The circuit network server 33 then transfers thecircuit-based signal to the circuit network 14.

[0032] In a preferred embodiment of the invention, the gateway containsmultiple circuit network servers 33 and multiple packet network servers36 with the packet switch fabric 34 coupling all the servers together sothat a signal may be transferred between any two servers. Routing maytake place on a circuit-based signal wherein the circuit-based signal istransferred from one circuit-based connection in the circuit network 14to a second circuit-based connection in the circuit network 14 as shownby line 37. This is done in the following fashion. The circuit-basedsignal is transferred into a circuit network server 33 where packetadaptation is performed forming a packet-based signal. The packet-basedsignal is transferred to the packet switch fabric 34 and then to asecond circuit network server 33. The second circuit network server 33performs packet adaptation on the packet-based-signal forming acircuit-based signal. The circuit-based signal is then transferred tothe circuit network 14 to the second circuit-based connection.

[0033] In a similar manner, a packet-based signal may be transferredfrom one address location in the packet network 16 to a destinationaddress. A packet-based signal is transferred to the packet networkserver 36. The packet-based signal is then transferred onto the packetswitch fabric 34. If transcoding is necessary, the packet-based signalis transferred to the signal processing server 35 where the packet-basedsignal is transcoded, and then sent to the packet switch fabric 34. Thepacket-based signal is transferred to a second packet network server 36and from the second packet network server 36 into the packet network 16.The packet-based signal is finally directed to the destination address.

[0034] The ability to perform packet adaptation prior to signalprocessing provides a great deal of flexibility in designing a gatewaysince there is no need for a circuit switch fabric in the gateway. Inone embodiment, the signal processing servers 35 are structured toperform all permutations of transcoding and are a shared resource amongall of the circuit network servers, so that the total number of digitalsignal processors performing transcoding is reduced. Since the number ofsignal processors that are required is decreased, the available space ona standard sized gateway layout board is increased allowing for agreater number of circuit network and packet network servers andtherefore a greater number of ports.

[0035] Although various exemplary embodiments of the invention have beendisclosed, it should be apparent to those skilled in the art thatvarious changes and modifications can be made which will achieve some ofthe advantages of the invention without departing from the true scope ofthe invention. These and other obvious modifications are intended to becovered by the appended claims.

What is claimed is:
 1. A system for connecting a packet network with acircuit network comprising: a module for receiving a packet-based signaland transcoding the packet-based signal creating a transcodedpacket-based signal; a module for receiving the transcoded packet-basedsignal, reassembling the signal creating a circuit-based signal,performing echo cancellation and transmitting the circuit-based signalto the circuit network; and a module for sending the transcodedpacket-based signal to the module for receiving the transcodedpacket-based signal.
 2. A system for connecting a circuit network with apacket network comprising: a module for receiving a circuit-based signaland performing echo cancellation and packet adaptation, creating apacket-based signal; a module for receiving the packet-based signal andtranscoding the packet-based signal creating a transcoded packet-basedsignal and sending the transcoded packet-based signal to the packetnetwork; and a module for transmitting the packet-based signal to themodule for receiving the packet-based signal.
 3. A system for connectinga circuit network with a packet network, the system comprising: a packetswitch fabric; a circuit network server having a first port for sendingand receiving circuit-based signals with the circuit network, thecircuit network server having a first at least one digital signalprocessor to perform packet adaptation and a second at least one digitalsignal processor which subsequent to the packet adaptation performssignal processing and a second port for sending and receivingpacket-based signals having packets with the packet switch fabric; and apacket network server having a first port for sending and receivingpacket-based signals with the packet switch fabric and a second port forsending and receiving packet-based signals with the packet network;wherein the packet switch fabric is capable of transferring packet-basedsignals among the packet network server and the circuit network server,and among the circuit network server and a second circuit networkserver.
 4. A system according to claim 3 wherein, the signal processingperformed on the second at least one digital signal processor is gatewaysignal processing.
 5. A system according to claim 4 wherein, the gatewaysignal processing on the second at least one digital signal processor ofthe circuit network server is transcoding.
 6. A system according toclaim 4 wherein, the gateway signal processing on the second at leastone digital signal processor of the circuit network server is echocancellation.
 7. A system according to claim 3 wherein, the packetswitch fabric further comprises a switch for switching among the packetnetwork server and the circuit network server.
 8. A system according toclaim 3 wherein, the packet switch fabric is a switching module.
 9. Asystem according to claim 3 wherein, the packet switch fabric is apacket bus.
 10. A system according to claim 3 wherein, the packet switchfabric is a cell bus.
 11. A system according to claim 3 furthercomprising a signal processing server having a port for sending andreceiving packet-based signals with the packet switch fabric, the signalprocessing server having a digital signal processor for performingsignal processing on the packet-based signals; wherein the packet switchfabric transfers packet-based signals to the signal processing server.12. A system according to claim 11 wherein, the signal processingperformed on the digital signal processor of the signal processingserver is gateway signal processing.
 13. A method for communicating acircuit-based signal as a packet-based signal, the method comprising:receiving a circuit-based signal into a circuit network server;performing echo cancellation on the circuit-based signal; performingpacket adaptation on the circuit-based signal forming a packet-basedsignal; transferring the packet-based signal to a packet switch fabric;transferring the packet-based signal from the packet switch fabric to asignal processing server; transcoding the packet-based signal creating atranscoded packet-based signal; directing the transcoded packet-basedsignal from the signal processing server to the packet network server;and sending the transcoded packet-based signal from the packet networkserver.
 14. The method of claim 13 wherein, said step of directingcomprises transferring the transcoded packet-based signal from thesignal processing server to the packet switch fabric and transferringthe transcoded packet-based signal from the packet switch fabric to thepacket network server.
 15. A system for connecting a circuit networkwith a packet network, the system comprising: a packet switch fabric; acircuit network server having a first port for sending and receivingcircuit-based signals with the circuit network, the circuit networkserver having a first at least one digital signal processor to performpacket adaptation and a second at least one digital signal processorwhich subsequent to the packet adaptation performs signal processing anda second port for sending and receiving packet-based signals havingpackets with the packet switch fabric; and a packet network serverhaving a first port for sending and receiving packet-based signals withthe packet switch fabric and a second port for sending and receivingpacket-based signals with the packet network; wherein the packet switchfabric is capable of transferring packet-based signals among the packetnetwork server and the circuit network server, and among the packetnetwork server and a second packet network server.
 16. A systemaccording to claim 15 wherein, the packet switch fabric is a switchingmodule.
 17. A system according to claim 15 wherein, the packet switchfabric is a packet bus.
 18. A system according to claim 15 wherein, thepacket switch fabric is a cell bus.